The Geochemical Behavior Of Rare Earth Elements In Water And Sediment Collected From Lower Reaches And Estuary Of The Minjiang River And Implications For Their Provenance

Rare earth elements,which have similar geochemical characteristic,are always acted as an important role in geochemical indicators,and were widely used in the "source to sink" researches.The Minjiang River is the biggest mountainous river in runoff in the coastal area of China,the annual runoff of whose was the third larggest of the Chinese rivers in 2010.The Minjiang River is characterized by big elevation gradient,strong hydrodynamic force and large material transport flux.Therefore,it is an ideal place to realize the REE geochemical behaviors in the mountain river.While,the researches on river material transport of Minjiang River are rare because of the historical causes.The geochemical researches of the Minjiang River are urgently needed.In order to realize the distribution of rare earth elements(REE)in waters and sediments and their influenced factors,and to clarify the character of land material in the transport path of mountainous river to the adjacent sea,we have collected 196 waters and 24 sediments in the Minjiang River Basin.In additional,34 physical and chemical indicators in waters and 52 chemical indicators in sediments,including concentrations of major,trace and rare earth elements,were analyzed.We have optimized the experimental conditions of seaFAST-ICP-MS pre-concentration system,and solved the problem of measuring the ultratrace elements in natural waters.The results show that the linearity coefficients of REE were in the range of 0.99967(Ce)to 0.99999(Eu,Gd,Tm);procedural blank values and DLs(3?)were below 0.5 ng/L.The reproducibilities,which were obtained by seventeen aliquots of 500 ng/L diluted REE standard solutions,were below 3%.The concentrations of three NRC(National Research Council of Canada)reference materials measured in this study were highly consistent with those values of previous researchers.The REE spike recovery show excellent performance.It is believable that the seaFAST-ICP-MS system can solve the problem of measuring the ultratrace REE concentrations in natural waters and the salinity influence.The ?REE of water collected in different seasons varied significantly from 825.65 to 5937.30 ng/L,and averaging in 2219.57 ng/L.The REE concentration in waters collected from the Minjiang River was higher than many of the world's major rivers,and was due to the parent rock,REE mineral reserves,climate and weathering.The REE concentration ratios and REE patterns of dissolved loads are characterized by progressing weaker LREE-enrichment and stronger HREE-enrichment from middle reaches to estuary.That was because of the slope of river path and human activities in lower reaches and of the land-ocean interaction in river mouth and the maximum turbidity zone.The average REE concentrations were descending from SHF,HF,NF and LF season.Comparing with NF and LF seasons,in which REE patterns are relatively flat,samples of SHF season have more LREE-enriched and HREE-depleted patterns that close to parent rocks,while samples of HF season are more LREE-depleted and HREE-enriched.Those seasonal differences could be attributed to the weathering,runoff and water-particle interactions.Almost all samples showed common features:slightly negative Ce and Eu anomalies,and a slightly positive Gd anomaly.REE fractionations in the waters between middle and lower reaches during SHF and HF seasons are stronger than NF and LF seasons.Generally,spatial and seasonal variations in REE abundance and pattern are presumably due to several factors,such as chemical weathering,mixture with rainfall and groundwater,estuarine mixing,runoff and biological production.REE concentrations decreased from salinity 0 ‰ to 3 ‰,and raised from salinity of 3 ‰ to 7 ‰,and decreased again in the higher salinity.The REE geochemical behaviors characters in waters were controlled by two land-ocean interaction processes,including physical mixture of runoff with tide and solid-liquid interface chemistry.The REE geochemical behaviors were differed in different seasons and even in different tidal period in the same season,because the strong water mixture in low flow season and saltwater wedge invasion in HF season.The REE concentrations in sediments of Minjiang River Basin were 230.1 to 420.2 ?g/g,averaging in 345.3 ?g/g,and the coefficient of variation was 16.1%.There were the common features of sediments collected in Minjiang River Basin:NASC normalized LREE enriched and HREE depleted.Comparing with the Changjiang,Huanghe and Zhujiang Rivers,the sediments of Minjiang River were transformed in the material transportpath.The REE patterns of those were affected by parent rock,climate,depositional condition and human and biological effects.The results of statistical analysis show that the REE and trace elements in sediments were influenced by provenance,human activity and redox indicators.Combining with the NASC normalized REE pattern,trace element normalized pattern,trace element ratios and results of statistical analysis,we found an efficient method in justify the provenance and deposition processes.